1. Field of the Invention
The invention relates to the field of optical communication, and more particularly to a multimode fiber suitable for access network or miniaturized optical apparatus, as well as to a method for producing the same. The fiber has excellent bending resistance.
2. Description of the Related Art
Multimode fibers, particularly those with high bandwidth, e.g., OM3, are widely used in short-medium distance optical fiber network system (such as data centers and campus networks) due to low cost of system construction. When used in indoor and narrow environments, particularly in a small storage box, fibers are exposed to great bending stress. Thus, to meet the requirements of the network construction and apparatus miniaturization, bending resistant multimode fibers are desired. In comparison with conventional multimode fibers, bending resistant multimode fibers are desired to possess the following properties. First, low additional bending loss, particularly, low additional macro-bend loss. The multimode fibers have a plurality of transmission mode, and high order mode transmitted close to the edge of the core is easily leaked out upon fiber bending. When the bending radius decreases, more photons are leaked out, and the system attenuation increases, thereby resulting in signal distortion and system error. Second, the service life of the multimode fiber should not be affected under low bending radius. Bending resistant multimode fibers may work at low bending radius for a long term. When the fiber bends, the outside thereof is exposed to tensile stress. The tensile stress is represented by the following formula:
  σ  =            E      ·      r              (              R        +                  C          th                +        r            )      wherein E represents young modulus of silica glass, R represents a bending radius, r represents the radius of a fiber, and Cth represents the thickness of a coating. For a fiber with a glass cladding diameter of 125 μm and an outer diameter of 250 μm, when the bending radius is decreased to 6.5 mm, the tensile stress imposed on the outer bending wall of the fiber is 0.69 GPa (100 kpsi), which reaches the common screening tension of fibers. Bending easily causes fracture, thereby increasing the building and maintenance cost and affects the reliability of the systems in the application of FTTx. Thus, bending resistant multimode fibers must have good mechanical properties so as to possess long service life under low bending radius. Compared with common multimode fibers, bending resistant multimode fibers should have smaller residual stress and fewer defects. Third, bending resistant multimode fibers should have high bandwidth so as to meet the transmission requirement of 10 Gb/s or even 40 Gb/s ethernet.
An effective method to improve bending properties of fibers is to design a depressed cladding, whose refractive index profile is a trench-type (as shown in FIG. 1) or a double cladding type (as shown in FIG. 2). The method is disclosed in US20080166094A1, US20090169163A1, and US20090154888A1. The principle is that when the fiber bends slightly, the photons leaked from the core are restricted in the inner cladding to a large extent and finally return to the core, and thereby the macro-bend loss decreases greatly.
How to ensure long service life of a fiber working for a long term at low bending radius is an urgent problem to be solved. The fiber whose refractive index profile is shown in FIG. 1 has a highly germanium doped core and a highly fluorine doped depressed cladding. The core and the depressed cladding are close to each other. The coefficient of expansion of silica glass doped with germanium is significantly different from that doped with fluorine. Thus, internal stress is produced inside the fiber. Although the additional bending loss caused by the internal stress can be solved by designing a depressed cladding, the service life of the fiber has been affected badly. In addition, when fiber bending, the internal stress causes the profile to distort, which affects the transmission bandwidth. The fiber whose refractive index profile is shown in FIG. 2 has the same the material composition as disclosed in the above mentioned US patents, but just like the fiber whose refractive index profile is shown in FIG. 1, the internal stress is still produced. The internal stress is originated from different thermal expansion coefficient of different layers. Thus, it is a permanent stress and hardly removed by improving the process, but can be removed by designing appropriate material composition and structure. The appendix of fiber standard ITU-TG.657 briefly describes the prediction of fiber life. The service life of fibers is related to the dynamic fatigue parameters (nd) thereof. Under identical bending radius and storage length, the higher the dynamic fatigue parameters of fibers, the higher the mechanical reliability thereof. Thus, the effect of upgrading the material composition and profile structure of the fiber can be determined by testing the dynamic fatigue parameters thereof.
To make a multimode fiber have good bandwidth, the refractive index profile thereof should be a close-to-perfection parabola. Some literatures including Chinese Patent No. 1183049C disclose methods for producing a preform with accurate refractive index distribution. However, in the process of fiber drawing, due to residual stress and the diffusion of compositions, the refractive index distribution of the resultant fiber may distort. That is to say, even if the refractive index distribution of a preform is a perfect parabola, that of a fiber drawn therefrom is not necessarily a perfect parabola.